Degenerative disc disease is a common condition of the intervertebral disc (IVD) of the spine characterized by disc height collapse with or without disc herniation, osteophyte formation, foramenal stenosis, facet hypertrophy, synovial cyst, and other symptoms. Any or a combination of these findings can lead to pain or neurological deficit. Many of the symptoms of degenerative disc disease may be alleviated by decompression of the neural structures and immobilization of the involved spinal segments. Immobilization is typically achieved in the long term by removal of the disc and placement of bone graft. Temporary immobilization to encourage incorporation of the bone graft can be achieved with placement of rigid hardware such as screws and rods.
While immobilization and a successful fusion may relieve the pain associated with nerve impingement, the long-term consequences of eliminating the motion of the IVD show a tendency toward increased risk of failure of the adjacent discs. The lack of motion at the fusion site places increased biomechanical demands on the adjacent discs causing them to degenerate prematurely.
Replacement prostheses have been suggested for degenerative disc disease to allow motion at the operative disc level. However these devices are devoid of stiffness and stability and rely on the remaining spinal elements, such as the ligaments, muscles and remaining IVD tissue, namely the annulus fibrosis, for stability. For example, U.S. Pat. No. 5,556,431 to Buttner-Janz, U.S. Pat. No. 5,507,846 to Bullivant and U.S. Pat. No. 5,888,226 to Chaim, all of which are incorporated herein by reference, describe prostheses that comprise ball and socket type joints. These inventions rely on stretching the annulus fibrosis to put the prosthesis into compression to gain stiffness. But there is risk of altering the spine's biomechanics by increasing the disc height past the normal range and risk of damage to the annulus fibrosis. If the disc space is not stretched enough an unstable spinal segment could result, possibly leading to pain and further injury. Furthermore, all of these prior art disc replacement prostheses consist of several parts that are not connected. Implantation entails insertion of several separate pieces that must be properly aligned during surgery. The surgery is often performed with a minimal incision offering limited access to the insertion site. Perfect alignment after insertion could be difficult.
Other prostheses have been suggested (for example, see U.S. Pat. No. 6,136,031 to Middleton, U.S. Pat. No. 5,320,644 to Baumgartner, U.S. Pat. No. 5,827,328 to Buttermann and U.S. Pat. No. 5,676,702 to Ratron, all of which are incorporated herein by reference) which have their own inherent stiffness, but do not take into account that axial loads placed on the spine during activity are generally much larger than bending loads. Therefore, these prostheses would either bottom out under axial loads and offer no response to bending loads, or be stiff enough to support the axial loads and thereby too stiff to flex under bending loads.
What is needed is an intervertebral disc prosthesis that assists in alleviating the symptoms of degenerative disc disease without sacrificing normal spinal mechanics.